So odd! W210 shows lamp out error when charging phone!
 

Yea, you read the title right. I've got a 1997 E300 diesel. I just got a new cigarette lighter phone charger with a separate USB charger cable for my Samsung S7. When I charge with it, and turn on any parking lights or head lights, the dash board displays a Lamp Malfunction warning. When I hit the little R button to reset the alert, it comes back on in a minute or so. I've not no lights out anywhere on the car. Anyone ever seen anything like it?

Here's the charger: https://www.amazon.com/gp/product/B07BX3D4PK/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Here's the USB cable: https://www.amazon.com/gp/product/B07CG8QZV1/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Does the cigarette lighter do the same thing with any other load attached, or is it just that charger that does it?

Exactly what I was thinking. It might be the circuitry in the charger is causing some interference or noise in the car's electrical system that is causing it to trigger the bulb warning lamp.

I've used this one in all my MB's with no issues (including a W210): https://www.amazon.com/Anker-Charger-PowerDrive-iPhone-Galaxy/dp/B00VH84L5E

The cigarette lighter does work, but I don't know if I've tried it with the lights on. I'll try that today.

I've tried chargers with the "nubbin" contacts on the side and they don't seem to stay securely in the lighter port. My Verizon brand charger now lives in my 2006 Jeep Wrangler for that reason. The one I got with the "strip" contacts on the side seems to have a snug fit in the W210 and never wiggles loose.

The 210 seems to have lots of grounding issues, I'd run a temporary ground to the socket and see what happens.

Many cars have moved to a shallow lighter and socket while power ports have kept the deep socket, this would explain the fall out.

These power adapters use an inverter where DC is chopped up at high frequency *, sent to a transformer then rectified into DC. ( Home power adapters rectify AC into DC then do the same as above. ) This process is electrically noisy and can cause problems if the car side of the adapter isn't properly filtered.

* High frequency is used because tranformers can be very small making for good packaging.

High intensity LED lights use the same technology and can have the same problems. In order to attain maximum brightness, current must be limited making an inverter the best choice.

Can you describe in idiot terms how this would be done? I'm comfortable changing brake pads, shocks, tie rods, and all sorts of physical things. I don't get electronics.

No problem.

Take a wire run it from the body, engine or battery negative and touch the other end to the nubbin / strip contacts or metal body of the lighter socket while your adapter is in use.

In doing this you are adding a second ground in addition to the one that already exists in an attempt to see if the original ground has a high resistance connection.

Not metal ( like dash brackets ) in a car is grounded so you may need to dig around a bit. Jumper cables from the negative side of the battery make for a good extension.

The bulb out indicator on my W210 was on for most of the twelve years I owned it. Black electrical tape is your best bet. Cut a three inch strip and apply to the info center display.